The invention relates to a method of manufacturing primers/detonators stable at high temperatures up to 100.degree. C. or better, having an all-fire impact sensitivity of 1.0 inch-oz or less, and high degree of reliability.
The invention more particularly relates to the development of a primer/detonator, which can function with a very high degree of reliability at temperatures as low as -40.degree. C., as high as 100.degree. C. or better, and function equally well at ambient temperatures, and should have an all-fire sensitivity to impact of 1 inch-oz or less in the aforementioned temperature range.
The explosive industry uses a variety of primers/detonators. Basically these devices consist of a primary explosive component initiated by stab (friction) or impact, an intermediate explosive composition to be set-off by the primer composition, and a base charge of secondary explosive like RDX or HMX to give the desired output to perform work, which may be to set off another explosive device in the ignition train. One of the common low input energy primers/detonators is an M55 Detonator, which is extensively used in ordnance for anti-personnel and anti-vehicular munition systems. The make up of these detonators consist of:
(a) A primary explosive composition containing basic lead styphnate, dextrinated lead azide, antimony sulfide, barium nitrate, and tetracene. PA1 (b) An intermediate explosive charge of RD 1333 lead azide. PA1 (c) RDX as secondary explosive. PA1 (i) The composition used in the primer should be easy to manufacture and capable of loading in automatic industry machines used for manufacture of primers/detonators. PA1 (ii) They should be safe for handling, particularly in systems using lead styphnate, where protection against static electricity may be an important safety factor. PA1 (iii) They should be thoroughly stable at temperatures as high as 100.degree. C. and should function reliably at temperatures as low as -40.degree. C.
These detonators/primers are set-off by stab action with a firing pin and show a sensitivity of about 0.80 inch-oz at 99.99% reliability and 95% confidence level. In this detonator system, while basic lead styphnate and dextrinated lead azide fill their role as the main primary explosives, barium nitrate fills the role as a supplier of oxygen to the system and the antimony sulfide as a fuel cum mechanical sensitizer, because of its high melting point. But it is tetracene that plays a unique and important role. It is a chemical sensitizer with the unique property that makes the system function at an input sensitivity or energy below 1 inch-oz.
While tetracene is an excellent sensitizer and one of the best which explosive chemists have developed, its inherent weakness is that when temperatures higher than 85.degree. C. are encountered, the primers begin to fail. With heat aging above 85.degree. C., the tetracene begins to decompose and leak out from the primer. Sensitivity starts to decrease at 95.degree. C., after 100 hours, the impact energy required will be increased by at least a factor of 3.
While there are many applications for primers/detonators that would function reliably at temperatures of 100.degree. C. or higher (like high cycle firing machine guns), a civilian application is in the automobile crash air bags used in motor vehicles for protecting occupants in crashes. In self-contained air bag modules involving mechanical sensors, the primers are used to ignite the propellant system, which then generates the gas to inflate the air bag. The industry standards demand that air bag systems function reliably at as high a temperature as 100.degree. C.; and at the same time function equally reliably at -40.degree. C. Also, industry standards demand that they function with a high degree of reliability and have a long shelf life.
The operating parameters expected for primers/detonators to fulfill the aforesaid, as well as similar requirements can be summed up as follows:
(iv) In systems using stab action energy to set off the system, the all-fire energy required for setting off the system should be 1 inch-oz or less, similar to those required for primers using tetracene as the sensitizer, where the all-fire sensitivity value is calculated statistically to 99.99% reliability and 95% confidence level for the entire population.